Before iOS8, I would occasionally encounter a problem where my iPhone and iPad would disappear from iTunes, and could only be recognized again by shutting down and starting up the devices again.
With iOS8, this started happening every day... shutting down / starting up the devices would make them show up in iTunes, and I could sync via WiFi, but then the next day, it wouldn't work (usually the devices would still be visible in iTunes, but when I tried to sync it would just spin the wheels for a while then the devices would disappear).
Since iOS8.1, I have not had to restart my devices like that... I've sync'd each day since Monday, and they are still showing up fine.
Hopefully that means the problem is fixed!